Sometimes attributed with magical powers or associated with the rightful sovereignty of Great Britain.
There are varing stories of the name of the sword and how King Arthur came to possess it.
Sometimes Excalibur and the Sword in the Stone are said to be the same weapon, but in most versions they are considered separate.
King Authur did not pull Excalibur for the stone but it was in give to Arthur by the lady of the lake.
More info can be found at http://en.wikipedia.org/wiki/Excalibur